Method and apparatus for coating a moving web of paper or board
The present invention relates to a method and apparatus for two-sided coating of a moving paper or board web with at least one coat layer.
In the coating of a paper web, to a base sheet manufactured in a paper machine is applied a coating mix layer that is smoothed to a desired thickness. The coating mix is made by slurrying coat solids into water and the web is dried after coat application prior to its entry into the subsequent finishing steps. The coater machine can be placed either directly after the paper machine manufacturing the base sheet, whereby the arrangement is called an on-line layout, or alternatively, as a separate section to which the wound base sheet rolls are transported to be unwound there and coated in an entirely separate off-line coater.
The production capacity of a coater machine is chiefly dictated by its width, web speed and reliability of the machine function. The most important factor affecting the functional reliability is the number of web breaks that should be kept as small as possible. Today, paper machines and coaters are already very wide, up to about 8-10 m, and a greater width is extremely difficult to achieve due to a number of reasons including the greater bowing of rolls. Hence, the productivity of paper machines and coaters is preferably improved by elevating the web speeds in these machines. However, at higher web speeds also the web run in the coater becomes more difficult to control. One major problem is caused by the boundary air layer travelling on the rapidly moving web that tends to detach the web from its support rolls. If the web loses its contact with the support rolls, control of web run becomes impossible and, respectively, loss of web contact with the backing roll at the applicator causes bagginess and problems in the application and smoothing of the applied coat. Hence, the control of web run in fast coating machines must be accomplished by means different from those used in prior-art machines. The most common technique is to use an at least partially supported web run through the machine. When the web is passed supported by an air-permeable wire or belt, the formation of an air layer between the web and the support wire or belt is prevented, thus allowing the web to stay in intimate contact with the surface of the support means. Further, a supported web run is an effective measure to reduce the number of web breaks due to variations in web tension, because tensioning of the web itself is not needed.
Instead of a wire, an air-cushion-type support means can be used in a coater for guiding the web run after coating, thus avoiding physical contact of any mechanical elements with the web. However, the air-cushion supported web guidance requires a substantially bulky free space particularly in the vertical dimensions of the machine, because the web cannot be guided by an air-cushion turning means in substantially sharp bends, particularly not through a sequence of sharp bends. Air-cushion-supported web run also needs dedicated means for threading the web trailing end through air dryers and air-cushion turning devices, because air dryers and air-cushion turning devices are incapable of pulling the web forward, but rather, the web must be drawn through the dryer units by means of pulling roll group or similar pulling means located after the dryer section and capable of maintaining a sufficiently tight web draw. However, this arrangement also involves the risk of web breaks due to variations in web tension.
In the manufacture of a coated paper grades, the current trend is to use a base sheet as thin as possible because the quality of the finished paper can be improved by coating in a better manner than by increasing the thickness of the base sheet and, moreover, the cost of the coat is appreciably lower than that of the base sheet. Obviously, the strength of a thinner base sheet is much lower and, hence, the risk of web breaks particularly when the base sheet becomes wet in application of the coating will be greater the thinner the base sheet that is used. Hence, the choice of a suitable application method is a particularly vital question in machines running at high web speeds and making thin paper grades. The optimal coating techniques for fast machines are such applications methods as film transfer application, jet application and spray application which impose a minimal stress on the web and cause a minimum penetration of coating mix and moisture into the web. Particularly advantageous herein is that these application methods at best can apply only so much coat to the web that no doctoring after application is required. Obviously, the stress of application on the web is thus minimized. When this kind of an application method causing a minimum stress on the web is combined with, e.g., an entirely belt- and wire-supported web run through the entire machine, the system can be made to operate extremely reliably even at high web speeds.
It is an object of the present invention to provide a method capable of permitting a substantial speed increase of a paper web coating machine, yet simultaneously keeping the vertical and machine-direction space requirements of the machine within reasonable dimensions.
The goal of the invention is achieved by way of advantageously using a film transfer coater for applying the coat to the web and then passing the web to a dryer cylinder group over a wire-supported air-cushion cylinder, thus making it possible to support the web over its entire run from application to the end of drying.
According to a preferred embodiment of the invention, the apparatus also includes a belt calender and a beltsupported winder, whereby the web is essentially supported over its entire run from the unwinder or paper machine exit end to the winder.
The invention offers significant benefits.
By virtue of the invention, the coater design can be made very compact and the web can be easily passed from the support elements of one paper machine section to the next. The web run is entirely supported except for a short length at the web tension measurement equipment, thus allowing the frequency of web breaks to be reduced to a minimum. The web is coated in a film transfer coater and then passed to a first dryer member which is a cylinder with an air-cushion function. On the air-cushion cylinder, the web surface is dried by air ejected through the cylinder shell, while the web is simultaneously supported by the wire and guided by its edges resting against the cylinder rims. Using belt supported top-side film transfer coaters, the coat can be applied to the upper surface of the web, whereby the length of web run is minimized. With the help of the air-cushion cylinder, the web can be passed via a very uncomplicated path to the dryer cylinders without marring the applied coat, yet keeping the length of web run at a minimum. The web is dried finally in a wire-supported dryer cylinder section and subsequently passed on the support belt of the next coater section. Here, the other side of the web is coated by film transfer applicators and the web is passed over an air-cushion cylinder to the next dryer cylinder section in the same manner as described above, whereby the construction and web run of the second coater/dryer section becomes very uncomplicated, too. The equipment may be very advantageously combined with a belt-supported calender and a belt-supported winder, whereby the web will run fully supported all the way starting from its entry to the coater and ending at the winder.